The only mention of an official Arduino WiFi Shield I can find is this blog post from September 17, 2011:http://arduino.cc/blog/2011/09/17/arduino-launches-new-products-in-maker-faire/
"Arduino Wifi Shield.
It adds Wi-Fi communication capabilities to any Arduino. Instead of using any of the classic WiFi modules on the market we wanted to have something that will provide the maximum level of hackability to the user. The shield is based on a wifi micro module made by H&D Wireless coupled with a powerful AVR32 processor that carries the full TCP-IP stack leaving room to add your own protocols and customisations. We’ve also worked hard to make sure that you will be able to migrate your code from the Ethernet Shield with minor changes."
(My guess is that the first step in getting in on Arduino WiFi Shield development is to move to Italy.)